Link two serial ports on linux (and sniff on the traffic)

I’m working on a system atm where one part communicates with the other over a (full duplex, no flow control) UART connection – so far nothing extraodinary. Normally when I need to see what’s happening in the communication between those two parts (especially if it’s timing related) I use a logic analyser and capture the traffic on both wires for later / live analysis.

But having the logic analyser not at hand, I used two UART to USB bridges (FTDI adapters) to connect my laptop to both parts, and then linked those two ports up (with added logging of the output) using socat:

socat -v /dev/ttyUSB0,rawer,b921600 /dev/ttyUSB1,rawer,b921600

The -v tells socat to write any transferred data also to stderr, but with some conversions for readability. And also with a prefix of > or < to indicate flow direction. One could also use -x to get the data as hex (with flow direction indication).

Usage of rawer was necessary in my case - most likely to avoid any conversions (line endings?) from happening. The b921600 is just the baudrate. socat of course also has options to specify things like parity and so on. Those options should be documented in man socat on any linux distribution.

Side note: This of course affects the timing of the communication line quite a bit! So not really an alternative if timing related issues have to be solved.